(1) Field of the Invention
This invention relates to pumps, and in particular to a pump having a piston and cylinder combination defining a pump chamber. The invention is especially concerned with such a pump arranged occasionally to deliver fluid such as lubricating oil on a single working stroke, rather than arranged for continuous operation.
(2) Description of the prior art
In an attempt to extract greater performance from internal combustion engines, and particularly diesel engines, it is now a common practice to provide an engine with an exhaust-gas-driven turbo-charger, so as to increase the volumetric efficiency of the engine. Turbo-chargers usually operate at very high rotational rates--typically of the order of 90,000 r.p.m.--and thus have stringent lubrication requirements. It is usual to supply lubricant to the bearings of the turbo-charger rotor from the engine lubricating oil system, but since the turbo-charger itself usually is some distance from the oil pump the engine lubrication system, there may be a considerable time lapse between the turbo-charger rotor starting to rotate and the delivery of oil to its bearings. Because the turbo-charger rotor starts to turn as soon as the engine first fires, the bearings of the rotor may be running relatively dry for some little while--typically 30 seconds, but in exceptional cases perhaps for 3 minutes.
This period of operation of the turbo-charger without a proper supply of lubricant has in the past frequently lead to the premature failing of the rotor bearings, and hence high operating costs on account of firstly the necessary repairs and secondly the out-of-service time of the engine. In an attempt to solve the above problem, I have proposed a design of single-shot oil pump for fitting to a turbo-charged engine, which pump performs a single oil-delivery stroke when the engine starter-motor is energised, so as to discharge oil directly to the bearings of the turbo-charger; subsequently on normal running of the engine, the pump is automatically re-charged with oil supplied by the engine lubrication system. Such a pump has been described and claimed in the Specification of my prior British Pat. No. 1,526,929.
Though the pump described in my prior Patent Specification largely eliminates the problem discussed above on starting a turbo-charged engine, nevertheless experience has shown that premature bearing failure can still occur within a turbo-charger. My investigations have shown that in addition to the starting problem, a somewhat similar situation can arise on stopping a turbo-charged engine. This is because the supply of lubricating oil under pressure to the turbo-charger bearings collapses almost immediately the engine stops, though the turbo-charger rotor may continue to turn for some considerable while thereafter. In view of the very high temperatures prevailing in a turbo-charger when operating, any remaining oil film can break down before the rotor also has stopped, thus leaving the bearings with no lubrication--and the problem is exacerbated if the engine is revved immediately prior to being stopped, because then the rotational rate of the rotor as the engine stops will be much higher.
One solution to the above-stated problem would be to provide a second single-stroke pump somewhat similar to that described in my prior British patent specification No. 1,526,929, but modified so that its working stroke is performed on the engine stopping. However, such a solution would be expensive to implement, because two separate pumps would have to be provided and moreover the space required to accommodate a second pump may not be available in the somewhat restricted area of an engine compartment, particularly in the case of a commercial vehicle.